The present invention relates to a process for producing a prepreg of a glass fiber cloth, particularly of a belt-shaped glass fiber cloth.
As is well known, prepregs are materials for laminates obtained by impregnating a continuous reinforcing material such as a fabric, a paper, a mat or the like with a thermosetting resin and then drying the impregnated material to cure the resin to B-stage.
The B-stage herein refers to a stage of a thermosetting resin wherein the resin swells but is not completely soluble in liquids such as an alcohol, acetone and the like and, when heated, the resin softens to a consistency close to that of rubber but does not melt completely. Thermosetting resins are generally used for impregnation of glass fiber cloth and the like in the form of varnish.
When prepregs are produced with a belt-shaped glass fiber cloth having fluffy selvages produced by a recent, improved loom such as an air jet loom or the like, the following problems arise.
The first problem is that an impregnated belt-shaped glass fiber cloth contains excessive resin at its selvages and the selvages tend to be thicker than other portions of the cloth. When such a prepreg having selvages of larger thicknesses is cut into pieces each of the same desired dimension and the pieces are laminated, the prepreg laminate has larger thicknesses at its selvages resulting in a laminate of non-uniform thickness. Thus, a prepreg not having a uniform thickness across the entire width brings about various inconveniences when the prepreg is processed into products. As an example, when the prepreg is used for production of a copper-clad laminate, application of a pressure across the entire prepreg width is impossible.
The second problem is that both selvages of a prepreg, when observed in the lengthwise direction of prepreg, are not linear but rugged due to fluffy selvages. When such a prepreg having uneven selvages is cut into pieces of the same desired length and the pieces are laminated, it is difficult to true up the edges of the resulting prepreg laminate and, when the edges are trued up, part of the rugged selvages is torn off, scattered and mixed into the laminate causing a poor quality product.
The third problem is that, during prepreg production, part of the selvages of a glass fiber cloth are torn off and mixed into the impregnation varnish and adhere to the surface of the prepreg causing a poor quality product.
The above are the problems occurring when a prepreg is produced from a belt-shaped glass fiber cloth having fluffy selvages. The first problem mentioned above should also occur with respect to selvages of a glass fiber cloth produced by a shuttle type loom.
In order to avoid the first and second problems mentioned above, selvages of a prepreg may be removed by cutting. This still leaves the third problem unsolved and further reduces yields of glass fiber cloth and varnish, which is not desirable from cost standpoint.
Selvages of a belt-shaped glass fiber cloth may be removed before the cloth is processed into a prepreg. However, no appropriate method has been developed. For example, in Japanese Laid-open Patent Application No. 34094/1977, there is proposed a method wherein, when a glass fiber cloth of wide belt shape is cut in the lengthwise direction to convert it into glass fiber cloths of narrower width, there are applied to the portions to be cut, for prevention of fraying, an adhesive of an acrylic, polyvinyl acetate or polyvinyl chloride resin or of a rubber, preferably an adhesive of a polyvinyl acetate resin compatible with polyester resins. However, it is doubtful whether or not the method is suitable for obtaining a glass fiber cloth of narrow width as a material for a prepreg. Further, even if a glass fiber cloth of narrow width has been produced according to this method and then its selvages have been removed, it is impossible to obtain a satisfactory prepreg free from the above mentioned problems. Also in Japanese Laid-open Patent Application No. 18568/1973, there is proposed a method wherein, when any knitted or woven cloth is cut along the stitch, the portions to be cut are impregnated with a heat-fusible synthetic resin for prevention of fraying. Even by application of this method, no satisfactory prepreg is obtainable.
An object of the present invention is to provide a prepreg of a glass fiber cloth which is free from the above mentioned drawbacks of conventional techniques, has a uniform thickness across the entire width and is linear and not rugged at both selvages and wherein fibers of selvage portions are not torn off and do not adhere to other prepreg portions so that there is no deterioration in quality.